This invention relates to improved anti-corrosion protective coatings for metal surfaces.
This invention further relates to improved anti-corrosion protective coatings for the surface of metal pipes that are destined for inground implantation.
The instant invention most particularly relates to helically-wrapped anti-corrosion protective coatings for the surface of metal pipes that are to be subjected to an inground high shear stress environment.
It has previously been the practice to provide anti-corrosion protective pipe coatings by supplying, in roll form, preformed polyolefin tapes, having one surface that is coated with a butyl-based adhesive, comprising a mixture of both virgin butyl rubber and reclaimed butyl rubber.
It has also been proposed previously to extrude both the polyolefin tape and the adhesive layer simultaneously, either separately, or as a coextrusion, directly onto the surface of a rotating pipe structure. This method, of course, is not suitable for over-the-ditch anti-corrosion protective tape-wrapping procedures.
Anti-corrosion protective coatings that are applied to inground pipeline structures are often subjected to rather severe long-term shearing forces derived from the surrounding soil. The magnitude of these shearing forces depends upon several factors, including amongst others: (a) the type of the soil, (b) the tectonic forces surrounding the implanted pipeline, (c) the size of the pipe, (d) the axial site emplacement and (e) the range of thermal expansion of the pipe as well as its contents.
In order to understand how each of the above factors affect the overall shear stress imparted to an inground pipeline coating, we first shall consider the forces acting upon implanted pipelines.
Frictional forces acting between the pipeline anti-corrosion protective coating and the surrounding soil are the primary source of shear stress. Frictional forces are here defined as the product of the frictional coefficient between the pipeline coating and the soil and the normal force acting around the pipe. As the coefficient of friction depends upon both the nature of the pipeline coating as well as the surrounding soil, it will be found to vary in different applications. Olefin polymer pipeline protective coatings, such as polyethylene, or the like, inherently exhibit lower coefficients of friction, as the protective tape outer surfaces are smooth and substantially non-adherent.
Other factors having importance in these considerations are the weight of the soil above the pipe, as well as the weight of the pipe, including its contents. In addition, since the normal force will vary depending on the axial position around the pipe diameter, the frictional force and hence the shearing force, will also be found to vary around the diameter of the pipe.
The result of long-term shear forces on a pipeline protective coating is referred to as "soil stress." Soil stress on anti-corrosion protective coatings generally results from the structural shear forces which cause the protective coating to creep along the pipeline peripheral surface.
Creep is, in essence, a long term visco-elastic, or "cold-flow" phenomenon, common to all polymeric substances. The amount of creep, however, will depend upon the physical properties of a coating. Since the physical properties (i.e. modulus) of a coating, will be temperature dependent, temperature becomes a decisive element in determining the amount of creep. At low temperatures, the propensity of the protective coating to creep will be substantially reduced, while at elevated temperatures, the likelihood of creep will be significantly increased, other factors remaining the same.
However, adhesive resistance to flow or creep, may be improved by introducing crosslinks between the component rubber chains.
When a rubber-based, or the like, adhesive system is crosslinked, (1) its resistance to creep is increased, (2) the overall dimensional stability is improved, and (3) it is more resistant to heat distortion. In addition, the above-listed crosslinking effects are generally intensified as the crosslink density is increased, and can therefore be controlled by adjusting the number of crosslinks in an adhesive coating. Crosslinking provides numerous anchoring points for the individual rubber chains, and these anchor points restrict excessive movement within the rubber of the adhesive, thereby resulting in limited creep or flow of the polyolefin tape coating.